1. Field of the Invention
The present invention relates to a grinding method for grinding a workpiece having a non-circular portion, such as a cam, or a circular portion (hereinafter a workpiece having a non-circular portion or circular portion may be referred to as a xe2x80x9cnon-circular or circular workpiecexe2x80x9d) by means of profile generation movement of a grinding wheel, and to a numerically controlled grinding machine which carries out the grinding method.
2. Description of the Related Art
Conventionally, a numerically controlled grinding machine is used to grind a non-circular workpiece, such as a cam, or a circular workpiece having a circular cross section and being eccentric from the rotational axis. In such a numerically controlled grinding machine, by use of a numerical controller, feed of a grinding wheel perpendicular to the axis of a main spindle for supporting the workpiece is controlled in synchronism with rotation of the main spindle. In order to effect synchronized control of the feed of the grinding wheel, profile data must be supplied to the numerical controller. The profile data include an amount of movement of the grinding wheel per unit rotational angle of the spindle which defines a reciprocation movement; i.e., profile generation movement of the grinding wheel along the finished or target shape of the workpiece.
In addition to the profile data, machining cycle data are also required in order to grind the workpiece. The machining cycle data are used to control a machining cycle which includes feed, cut-in feed, and retraction of the grinding wheel. The workpiece is ground on the basis of the machining cycle data and the profile data. In such a grinding operation, the relation between a back-off movement of the grinding wheel and the profile generation movement of the grinding wheel after completion of grinding is very important for attaining high grinding accuracy and high grinding speed.
Due to limited functions of the conventional grinding machine, when the grinding wheel is to be retracted after completion of grinding, the grinding machine must be operated in the sequence of stopping rotation of the main spindle and then retracting the grinding wheel rapidly. However, if the rotation of the main spindle is stopped while the rotating grinding wheel remains in contact with the workpiece, the workpiece is pressed against the grinding wheel by means of a so-called spring-back effect of the mechanical system, with the result that a surface of the workpiece in contact with the grinding wheel is ground and a depression is formed on the contact surface.
In view of the foregoing, an improved numerically controlled grinding machine which can solve the above-described problem has been proposed (see Japanese Patent Publication (kokoku) No. 6-41095. In the improved numerically controlled grinding machine, back-off data for controlling back-off movement of the grinding wheel after completion of spark-out are combined with profile data within a predetermined angle range defined on the workpiece, in order to superpose the back-off movement on the profile generation movement, whereby the grinding wheel is caused to effect back-off movement without stoppage of the main spindle.
The principle of the grinding method will be described with reference to FIG. 1.
FIG. 1 shows a locus of movement of a grinding wheel relative to a non-circular workpiece when the workpiece is ground by use of a numerically controlled grinding machine. Reference letter O denotes the axis of a main spindle; W denotes the non-circular workpiece; and G denotes the grinding wheel. Since the grinding wheel G reciprocates along an X direction in synchronism with rotation of the workpiece W in a xcex8 direction, when viewed in a coordinate system fixed to the workpiece W, the grinding wheel G revolves around the workpiece W in a direction of arrow A. During rough grinding, fine grinding, and finish grinding steps, cut-in advancement movements d1, d2, and d3 are carried out, respectively, in a section extending over a rotation angle xcex82. In FIG. 1, broken lines indicate the outer diameters of the workpiece W before the cut-in advancement movements d1, d2, and d3; and chain lines indicate the positions of the grinding wheel G before the cut-in advancement movements d1, d2, and d3. Reference letter L denotes a locus of the center of the grinding wheel G when the grinding wheel G carries out the profile generation movement relative to the workpiece W (during spark-out).
The grinding method employed in the above-described grinding machine carries out, without stopping the main spindle, the profile generation movement and the back-off movement after completion of grinding in parallel. That is, during spark-out, the grinding wheel G moves along the locus L in order to generate a profile on the workpiece W, and the profile generation (spark-out) is ended at point P1. Subsequently, the grinding wheel G is fed along a curved line extending from point P1 to point P2, whereby the grinding wheel is retracted within the section of the rotational angle xcex81. In this section, the profile generation movement and the back-off movement are performed concurrently. Subsequently, if necessary, the main spindle is stopped at point P2, and the grinding wheel G is retracted to point P3 at high speed.
Specifically, after completion of grinding, data for defining the back-off movement are supplied from data setting means and are combined with previously supplied profile data by data combining means. The data combining is performed in such a manner that the back-off movement is superposed on the profile generation movement; i.e., in such a manner that the grinding wheel G moves along the curved line extending from point P1 to point P2. The grinding wheel back-off means controls the position of the grinding wheel on the basis of the combined data and in accordance with the rotation angle of the main spindle.
Although the above-described grinding wheel back-off means solves the problem of a depression being formed on the workpiece upon completion of grinding, the conventional grinding method has a drawback of requiring a long machining time, because all of the conventionally employed grinding steps, including rough grinding, fine grinding, finish grinding, and spark-out grinding, must be performed without omission.
Accordingly, an object of the present invention is to provide an improved grinding method for grinding non-circular or circular workpieces which can avoid the problem of a depression being formed on a workpiece upon completion of grinding and which can shorten machining time.
Another object of the present invention is to provide an improved numerically controlled grinding method which carries out the grinding method.
In order to achieve the first object, the present invention provides a method for grinding a circular or non-circular workpiece in a plurality of grinding steps, the method comprising: causing a grinding wheel to effect profile generation movement in synchronism with rotation of the workpiece and in accordance with profile data derived from the target shape of the workpiece; advancing, in each grinding step, the grinding wheel in such a manner that the grinding wheel causes cut-in movement within a predetermined cut-in angle defined on the workpiece; and retracting, after completion of a final finish grinding step, the grinding wheel over a predetermined back-off angle defined on the workpiece, the retraction being effected in accordance with composite data obtained through combining the profile data and back-off data, the back-off angle being greater than the cut-in angle employed during the final finish grinding step.
Since the grinding method according to the present invention can eliminate spark-out grinding, which has conventionally been performed after final finish grinding, required machining time can be shortened.
Preferably, the cut-in angle employed during the final finish grinding is not greater than one-third the back-off angle. Preferably, the cut-in angle is decreased stepwise toward the final finish grinding step.
Although the above-described effect is attained insofar as the cut-in angle during the final finish grinding step is smaller than the back-off angle, the workpiece can be machined to high accuracy without fail when the cut-in angle during the final finish grinding is not greater than one-third the back-off angle and/or when the cut-in angle is decreased stepwise toward the final finish grinding step.
In order to achieve the second object, the present invention provides a numerically controlled grinding machine for grinding a circular or non-circular workpiece in a plurality of grinding steps, the grinding machine comprising: a movement mechanism for moving a grinding wheel relative to the workpiece; a storage unit for storing profile data derived from the target shape of the workpiece and defining profile generation movement of a grinding wheel to be performed in synchronism with rotation of the workpiece, machining cycle data defining at least a cut-in feed amount and a cut-in angle to be used in each grinding step, and back-off data defining at least a back-off angle to be used in a back-off step; and a control unit connected to the movement mechanism and the storage unit. The control unit causes the grinding wheel to effect profile generation movement in synchronism with rotation of the workpiece and in accordance with the profile data; advances, in each grinding step, the grinding wheel in such a manner that the grinding wheel undergoes cut-in movement within a corresponding cut-in angle; and retracts, after completion of a final finish grinding step, the grinding wheel over the back-off angle, the retraction being effected in accordance with composite data obtained through combining the profile data and back-off data, the back-off angle being greater than the cut-in angle employed during the final finish grinding step.
Since the grinding machine according to the present invention can eliminate spark-out grinding, which has conventionally performed after final finish grinding, required machining time can be shortened.
Preferably, the control unit decreases the cut-in angle stepwise toward the final finish grinding step. In this case, since the volume of an unground portion left after completion of the final finish grinding decreases, the required machining time can be shortened further, and more accurate grinding is enabled.